1. Field of the Invention
The present invention relates to an optical recording medium such as an optical disk, an optical card, and an optical tape, in which molecular rotation, or state changes such as molecular agglomeration and arrangement of a polymer compound chain are caused, and a light-absorbing compound causes association and dissociation with respect to the chain to largely change mutual optical characteristics between the two compounds, thereby amplifying information and reversibly repeating recording/erasure thereof.
2. Description of the Related Art
Recently, as laser techniques have progressed, optical recording media for writing, reading, and erasing information by using laser light have been extensively studied. A recording mechanism of such an optical recording medium is classified into (1) a photon mode type (former) which utilizes photons of light, and (2) a heat mode type (latter) which utilizes light as heat.
The former has attracted attention as a high-sensitivity and high-speed optical recording medium and was proposed as an optical recording medium consisting of a photochromic material such as spiropyrane [Published Unexamine Japanese Patent Application No. 61-17037, the 50th Japan Chemical Society Spring Annual Meeting Drafts Vol. 1, P. 259(85)], fluguide, indigo [Published Unexamined Japanese Patent Application No. 61-128244, J chem Soc, Perkin Trans, 1, 197(81), 2,341(81)], and diallylethene (Published Unexamined Japanese Patent Application No. 63-24245). Although this medium has the above characteristics, it causes a reverse photochromic reaction or causes a side reaction upon writing since a photochromic reaction is used in both writing and reading. In addition, since the medium has only a low light resistance with respect to reproducing light, its stability and repeating property in a color producing state are poor. Furthermore, this optical recording medium has a poor storage stability, i.e., it has a serious problem in reliability and the like as an optical recording medium. Therefore, the optical recording medium of this type is far from practical applications although it has been extensively studied.
The latter heat mode type optical recording medium causes local melting, evaporation, thermal deformation, thermal transition, and the like when recording light is radiated on a recording layer, and performs recording and reproduction by using the obtained optical change. A conventional example of the optical recording medium of this type is a photomagnetic recording medium consisting of a magnetooptic material, e.g., TbFe, GdFe, TbFe, or TbFeCo, which utilizes a magnetooptic effect such as a Faraday effect or a Kerr effect. Since, however, it is difficult to perform high-speed overwrite by using this medium, an inorganic optical recording medium consisting of, e.g., TeOx, Te-Ge, Te-Ge-Sb, and Te-Ge-Sb-Tl containing a calgon element, a typical element of which is Te by Ovshibsky, has been conventionally proposed. Since, however, the optical recording medium consisting of such an inorganic material is manufactured by sputtering a multi-metal compound by using a noxious material, its stability is low. Therefore, it is difficult to obtain a non-toxic and inexpensive optical recording medium having high reliability. In addition, in terms of medium characteristics of this optical recording medium, recording sensitivity is low, and stability, high-speed erasability, and overwrite performance in a recording state are insufficient. Therefore, a demand has arisen for a medium which can be manufactured by a simple method, which is non-toxic and inexpensive, and which has high reliability, a high speed, and high sensitivity.
In order to eliminate the above drawbacks, an optical recording medium consisting of an organic polymer material has been extensively studied since it is non-toxic, can be manufactured at low cost, and has high sensitivity. Examples of the optical recording medium of this type are recording performed by repeating pit formation-flattening using the above photochromic material or a thermoplastic polymer and a near-infrared absorbing dye (Published Unexamined Japanese Patent Application No. 58-48245) and phase separation of a polymer blend. These examples, however, have poor sensitivity and repeating property. A cell structure obtained by sealing a polymer liquid crystal between a pair of electrodes is disclosed as an optical recording medium utilizing phase transition as a recording principle and using a liquid crystal material in some conventional methods (Polym Commum. 24,364, 1983, Published Unexamined Japanese Patent Application Nos. 59-10930, 60-114823, and 60-166481). In these methods, however, electrodes are required, and a medium arrangement is complicated. In addition, a S/N ratio and a speed are insufficient. Examples of the polymer liquid crystal are a system applying a backbone liquid crystal polymer (Published Unexamined Japanese Patent Application Nos. 61-28004 and 62-175939) and a side chain liquid crystal polymer (Germany, Oftn DE3500838 Al) obtained by a method in which a liquid crystal polymer described in Macromolecules, 17, 1004 to 1009, 1984, Advanced in Polymer, 59, 37, 1984 is utilized and a domain layer of light scattering is formed into an isotropic layer by heat generated by light radiation and immobilized, and a system in which an absorber is further added to the polymer (Published Unexamined Japanese Patent Application No. 58-125247). This optical recording medium is, however, unsatisfactory in recording sensitivity, contrast, a repeating property, and recording storage stability, and cannot be put to practical use as an optical recording medium in terms of a response characteristic. In order to improve the response characteristic, a method is proposed (Published Unexamined Japanese Patent Application No. 63-98852) in which a photochromic molecule such as azobenzene is bonded to a side chain of polypeptide having lyotropic liquid crystal layer utilizing a relationship between a liquid crystal and an optical reaction or is mixed with the polypeptide, thereby utilizing photoisomerism. This method, however, has difficult practical problems in contrast, a repeating property, and memory storage stability as a recording medium since it has sensitivity at a shorter wavelength than a semiconductor wavelength which is industrially utilized. Therefore, no satisfactory recordable/erasable optical recording medium has been obtained.